1. Field of the Invention
The present invention is related to molds and methods for making same useful in casting single crystal metallic articles.
2. Description of the Prior Art
In casting single crystal metallic articles, a mold having a crystalline growth zone or cavity at the lower end and a narrow passage above and in communication with the growth zone is widely used such as, for example, in the Piearcey patent, U.S. Pat. No. 3,494,709 of common assignee herewith. Generally, the mold growth zone has an open bottom so that when the mold is placed on a chill plate during casting, the molten metal therein will be unidirectionally solidified to cause columnar grain growth toward the passage. The narrow passage functions to select a single crystal from among the numerous crystals growing upwardly in the growth cavity for further propagation through the mold. In the past, it has been common for the passage to take the form of a helix of small cross-section as compared to the growth cavity, such a passage being illustrated in U.S. Pat. Nos. 3,625,275; 3,627,015; 3,667,533; 3,690,368; 3,700,023 and 3,712,368, all of common assignee with the present invention.
In one prior art practice, the molds used in single crystal casting are made by the well known shell molding or "lost wax" process in which layers of ceramic material are deposited on a disposable pattern such as a wax pattern. The wax pattern generally comprises an article portion, a helix portion and a growth zone portion, the helix portion being wax welded by hand between the others. It is the assembled wax portions which are then covered with the ceramic layers to form a shell mold therearound. After the desired thickness of ceramic shell is deposited, the wax pattern is melted out, leaving behind a ceramic shell mold having an upper article cavity connected to a lower growth cavity by a helical passage of small cross-section.
As a result of the small cross-section and consequent structural weakness of the wax helix, problems have arisen during assembly of the wax pattern portions and during shell molding in the form of distortion or breakage of the helix and other wax portions. This problem requires operating personnel to use utmost care in these operations; however, notwithstanding such care, distortion and breakage of the helix occurs all too frequently and results in lower production and increased casting costs.
In another prior art practice, molds for casting single crystals are provided by assembling a precast central mold element or strongback between precast outer mold elements as shown in detail in the Hayes and Phipps patent, U.S. Pat. No. 3,965,963, of common assignee herewith. In this method, the mold elements are preformed or precast by injecting ceramic slurry into a suitably configured molding cavity.